* i2c algo bit timeout question
@ 2009-04-22 23:19 Dave Airlie
2009-04-23 6:57 ` Jean Delvare
0 siblings, 1 reply; 3+ messages in thread
From: Dave Airlie @ 2009-04-22 23:19 UTC (permalink / raw)
To: LKML; +Cc: Jean Delvare
Hi to any i2c people,
So I've been debugging some EDID fetching failures and wanted to ask
about the use of time_after_eq in the i2c bit banging code.
EDID specification recommends 2ms timeout for the ack on the initial
read, so we set the timeout in our code to usecs_to_jiffies(2200) (10%
margin of error). On my systems this ends up as 1, and we seem to fail
to retrieve EDID one in 10-20 times. Changing the value to 2, always
gets me the EDID I want.
So looking at drivers/i2c/algos/i2c-algo-bit.c it appears it uses
time_after_eq on jiffies, start + timeout value. So if we have a 10ms
jiffie resolution and enter this at the 9ms point in the 10ms window,
we will seem to exit the loop after 1ms instead of the minimum which I
asked for which is 2.2ms. Should this code use time_after instead of
time_after_eq?
or am I missing something else.
Dave.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: i2c algo bit timeout question
@ 2009-04-23 6:57 ` Jean Delvare
0 siblings, 0 replies; 3+ messages in thread
From: Jean Delvare @ 2009-04-23 6:57 UTC (permalink / raw)
To: Dave Airlie; +Cc: LKML, Linux I2C
Hi Dave,
On Thu, 23 Apr 2009 09:19:21 +1000, Dave Airlie wrote:
> Hi to any i2c people,
i2c people tend to live on the linux-i2c list, Cc'd.
> So I've been debugging some EDID fetching failures and wanted to ask
> about the use of time_after_eq in the i2c bit banging code.
>
> EDID specification recommends 2ms timeout for the ack on the initial
> read, so we set the timeout in our code to usecs_to_jiffies(2200) (10%
> margin of error). On my systems this ends up as 1, and we seem to fail
> to retrieve EDID one in 10-20 times. Changing the value to 2, always
> gets me the EDID I want.
>
> So looking at drivers/i2c/algos/i2c-algo-bit.c it appears it uses
> time_after_eq on jiffies, start + timeout value. So if we have a 10ms
> jiffie resolution and enter this at the 9ms point in the 10ms window,
> we will seem to exit the loop after 1ms instead of the minimum which I
> asked for which is 2.2ms. Should this code use time_after instead of
> time_after_eq?
Yes, I think it should. This bug has been there pretty much since
forever. I suppose people didn't notice because they usually use a
large timeout value.
Please send a patch fixing this and I'll apply it.
--
Jean Delvare
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: i2c algo bit timeout question
@ 2009-04-23 6:57 ` Jean Delvare
0 siblings, 0 replies; 3+ messages in thread
From: Jean Delvare @ 2009-04-23 6:57 UTC (permalink / raw)
To: Dave Airlie; +Cc: LKML, Linux I2C
Hi Dave,
On Thu, 23 Apr 2009 09:19:21 +1000, Dave Airlie wrote:
> Hi to any i2c people,
i2c people tend to live on the linux-i2c list, Cc'd.
> So I've been debugging some EDID fetching failures and wanted to ask
> about the use of time_after_eq in the i2c bit banging code.
>
> EDID specification recommends 2ms timeout for the ack on the initial
> read, so we set the timeout in our code to usecs_to_jiffies(2200) (10%
> margin of error). On my systems this ends up as 1, and we seem to fail
> to retrieve EDID one in 10-20 times. Changing the value to 2, always
> gets me the EDID I want.
>
> So looking at drivers/i2c/algos/i2c-algo-bit.c it appears it uses
> time_after_eq on jiffies, start + timeout value. So if we have a 10ms
> jiffie resolution and enter this at the 9ms point in the 10ms window,
> we will seem to exit the loop after 1ms instead of the minimum which I
> asked for which is 2.2ms. Should this code use time_after instead of
> time_after_eq?
Yes, I think it should. This bug has been there pretty much since
forever. I suppose people didn't notice because they usually use a
large timeout value.
Please send a patch fixing this and I'll apply it.
--
Jean Delvare
^ permalink raw reply [flat|nested] 3+ messages in thread
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